1. Field of the Invention
The present invention relates to a method for purifying a polyphenylene ether which is produced by oxidative polymerization of a 2,6-di-substituted-phenol.
A polyphenylene ether which is a polymer produced by oxidative polymerization of a 2,6-di-substituted-phenol has excellent mechanical properties, electrical properties, heat resistance and the like and also absorbs water poorly. Hence there has been an intense interest to use the polyphenylene ether as a thermoplastic engineering plastic.
This polyphenylene ether is generally produced by oxidative polymerization of a 2,6-di-substituted-phenol in an organic solvent. The polyphenylene ether is recovered from the polymerization solution and purified by bringing the polymerization solution in contact with a poor solvent such as methanol. The used catalyst is extracted or decomposed at the same time or before this operation is conducted.
2. Description of the Prior Art
Several methods for extracting or decomposing the catalyst have been proposed. It has been proposed to employ inorganic acids such as hydrochloric acid or organic acids such as acetic acid as a solvent for extracting or decomposing the catalyst in U.S. Pat. No. 3,630,995. It has been proposed that chelating agents such as a ethylenediaminetetraacetic acid (EDTA) and the like are added to a reaction product obtained by a solution polymerization method in U.S. Pat. Nos. 4,058,504, 3,838,102, 4,237,265, 4,110,311, 4,026,870, 3,951,917, 4,088,634, 4,116,939, 4,654,418 and 4,460,764. It has been proposed further that a reaction product obtained by a precipitation polymerization employing a specific polymerization medium is washed with chelating agents such as EDTA and the like and water in U.S. Pat. No. 4,463,164.
However, the method employing hydrochloric acid or acetic acid has the disadvantage that the obtained polymer is remarkably discolored during heat molding of the polymer. The method, in which chelating agents such as EDTA and the like are added to a reaction product obtained by a solution polymerization method, decreases the intrinsic viscosity of the polyphenylene ether during the operation of purification.